During this decade, more than 1.5 million women in the United States will be newly diagnosed with invasive breast cancer, and about 30% of them will ultimately die of the disease. Two thirds of the breast cancers are manifested in the post-menopausal period. Cancer prophylaxis trials for tamoxifen are currently underway on groups of women at high risk for developing breast cancer. By the year 2000, it is possible that many hundreds of thousands of women will be taking tamoxifen for a long period of time, and it is imperative that we know as much as possible about the mechanism of action of the drug. The long range goal of our research effort is to raise the basic level of knowledge on the molecular mechanism underlying the uterotrophic effects commonly seen with tamoxifen therapy in order to understand the association between tamoxifen therapy and endometrial neoplasia. Our working hypothesis is that tamoxifen-induced pathological changes in the uterine endometrium are due to dysregulation of expression of the genes which control cell proliferation and apoptosis. We will carry out a detailed analysis of the molecular effects of tamoxifen and 4-HPR (fenretinide; a retinoic acid derivative) on the uterine endometrium. The development of 4-HPR combined with tamoxifen as a breast cancer chemoprevention strategy is a priority at NCI. However, the effect of this treatment on the uterus has not been clearly established. We will determine the effect of these chemotherapeutics on the expression of mRNAs and proteins for estrogen-regulated genes in the uterus, such as protooncogenes and growth factors. This information should provide clues as to how and why tamoxifen is associated with endometrial neoplasia and if 4-HPR can modulate the uterotrophic effect of tamoxifen. Dysregulation of apoptosis has been proposed to contribute to pathogenesis of neoplasms, including breast cancer. To test our hypothesis that dsyregulation of apoptosis is a causative event in the evolution of tamoxifen-associated uterine pathologies, we will determine if expression of p53, bcl-2 and bax is altered during long-term tamoxifen and 4-HPR treatment. It is the paradoxical estrogen agonist activity of tamoxifen that causes the high prevalence of pathological endometrial changes associated with tamoxifen therapy. If more is known about the drug's mechanism of action in uterine cells, it may be possible to avoid this undesirable side effect. We will use the two-hybrid system to determine if uterine cells contain specific protein(s) or co-activator(s) that mediate the transcriptional activity of the tamoxifen-activated estrogen receptor. This will serve as our first step toward understanding how transcriptional activity in uterine cells is stimulated by tamoxifen in vivo. An understanding of the mechanism of action of tamoxifen and 4-HPR on uterine epithelia is of interest and of the utmost relevance to the evaluation of tamoxifen as a chemopreventive regimen for breast cancer.